linux-kernel - Re: [PATCH v6 4/4] x86: mm: support ARCH_MMAP_RND

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Date:	Mon, 14 Dec 2015 12:51:51 -0800
From:	Daniel Cashman <dcashman@...roid.com>
To:	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Cc:	linux@....linux.org.uk, akpm@...ux-foundation.org,
	keescook@...omium.org, mingo@...nel.org,
	linux-arm-kernel@...ts.infradead.org, corbet@....net,
	dzickus@...hat.com, ebiederm@...ssion.com, xypron.glpk@....de,
	jpoimboe@...hat.com, kirill.shutemov@...ux.intel.com,
	n-horiguchi@...jp.nec.com, aarcange@...hat.com, mgorman@...e.de,
	tglx@...utronix.de, rientjes@...gle.com, linux-mm@...ck.org,
	linux-doc@...r.kernel.org, salyzyn@...roid.com, jeffv@...gle.com,
	nnk@...gle.com, catalin.marinas@....com, will.deacon@....com,
	x86@...nel.org, hecmargi@....es, bp@...e.de, dcashman@...gle.com,
	arnd@...db.de, jonathanh@...dia.com
Subject: Re: [PATCH v6 4/4] x86: mm: support ARCH_MMAP_RND_BITS.

On 12/14/2015 10:58 AM, H. Peter Anvin wrote:
> On 12/11/15 09:52, Daniel Cashman wrote:
>> diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
>> index 844b06d..647fecf 100644
>> --- a/arch/x86/mm/mmap.c
>> +++ b/arch/x86/mm/mmap.c
>> @@ -69,14 +69,14 @@ unsigned long arch_mmap_rnd(void)
>>  {
>>  	unsigned long rnd;
>>  
>> -	/*
>> -	 *  8 bits of randomness in 32bit mmaps, 20 address space bits
>> -	 * 28 bits of randomness in 64bit mmaps, 40 address space bits
>> -	 */
>>  	if (mmap_is_ia32())
>> -		rnd = (unsigned long)get_random_int() % (1<<8);
>> +#ifdef CONFIG_COMPAT
>> +		rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_compat_bits);
>> +#else
>> +		rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
>> +#endif
>>  	else
>> -		rnd = (unsigned long)get_random_int() % (1<<28);
>> +		rnd = (unsigned long)get_random_int() % (1 << mmap_rnd_bits);
>>  
>>  	return rnd << PAGE_SHIFT;
>>  }
>>
> 
> Now, you and I know that both variants can be implemented with a simple
> AND, but I have a strong suspicion that once this is turned into a
> variable, this will in fact be changed from an AND to a divide.
> 
> So I'd prefer to use the
> "get_random_int() & ((1UL << mmap_rnd_bits) - 1)" construct instead.

Good point.  Will change in v7 across patch-set.

Thank You,
Dan

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